The invention relates to a washing machine comprising a tub, a solenoid valve for the water feed to the tub, a discharge pipe, a delivery pipe, a pump and a valve preferably of flap type for detergent economy located at the mouth of the discharge pipe at the tub.
The invention further relates to a control device suited for use in a washing machine.
A method has been known for some time which on washing machine start-up enables the flap valve to be closed by the effect of a pressure exerted on said valve by a water head present in the discharge and delivery pipes. Closing this valve is known to prevent detergent passing partly into the discharge pipe and pump during its feed into the tub, with consequent wastage. There is therefore the need on washing machine start-up to create a water head in the discharge and delivery pipes which is sufficient to close the valve and keep it closed. Various control devices have already been designed and constructed for operating the jump pulse-wise to obtain the necessary head. In particular, these devices comprise the use of a resistor the resistance of which varies directly as a function of its temperature, namely a PTC, which has negligible electrical resistance at low temperature, but high resistance at high temperature, this high temperature being obtained by the passage of current through the PTC itself. One of such devices uses a PTC in parallel with a solenoid valve which is connected in series with a timer and pressure switch. On washing machine start-up this circuit acts by way of the PTC on the pump to operate it for a short time period (eg. 5-15 seconds) so that the pump feeds into the delivery pipe the water which has remained in the machine after the previous wash. As the PTC heats up on passage of current through it, its resistance becomes very high and the current then passes through the branch circuit in which the solenoid valve is connected, this then operating to feed water into the tub to a level determined by the pressure switch. This causes a voltage drop across the solenoid valve such that the voltage across the pump is finally too low to operate it. When pump operation ceases, the water present in the delivery pipe flows towards the tub, so closing the flap valve. The water head thus created acts on the valve to keep it closed.
Although this method has various advantages (the flap valve operation takes place in a practical manner and the circuit can be constructed very simply at low cost), it has a serious drawback in that whenever the washing machine is stopped during the wash stage, its subsequent restart takes place with the PTC cold (and therefore with low resistance), with the result that the current is able to pass through it and operate the pump so that the valve opens and the water and detergent present in the tub are discharged.
Various alternative forms of the aforesaid circuit have been devised, but these all retain the said circuit drawback.